Design Ideas: July 7, 1994
A simple method for directly measuring an IC's temperature uses the device's input-protection diodes as temperature sensors. Static-discharge protection diodes are common features in many modern IC input structures, and their location on the IC makes them ideal as noninvasive temperature sensors.
Fig 1 shows the typical input structure for a CMOS IC and the measurement setup. One protection diode is forward-biased with a current source and suitable supply voltage greater than VDD. The device under test can be fully operational if the selected input is normally tied to logic HIGH. For a logic LOW input, substitute a current sink and find a voltage supply less than VSS. Other IC process technologies may also have accessible diode structure.
The ideal diode equation
V=(nKT/q)*(I/IO+1))
indicates the diode's temperature dependence. In practice, the voltage-temperature relationship is not quite linear, particularly over a large temperature range. Forward-bias currents from 10 µ:A to 1 mA have worked successfully with diode temperature sensors. Using the higher end of this range is recommended to avoid errors from device input current and the VDIODE meter impedance; use lower bias current when the IC's input structure includes a significant series resistance. The performance of the IBIAS constant-current generator is not critical. It is best to measure the volts-vs-temperature (V-T) slope of at least one representative device rather than relying on the above equation alone because of process-related departure from ideal behavior.
For a given process lot, the slope of the diode's V-T curve demonstrated consistent temperature-rise measurements within 1 or 2 degrees C over a 100 degrees span. Slope values of -2 to -2.4 mV/degreesC are typical for silicon. The intercept point on the V-T curve is less repeatable among devices, so if only lot sample data is available, a second measurement at a known temperature is recommended (ie, room temperature with the device's power off).
A word of caution: older CMOS ICs suffer from parasitic SCR latch-up when the input-protection diodes are driven into conduction. The results of this condition can be catastrophic. Modern CMOS designs are free from latch-up, with input diodes conducting tens of milliamps or more.
Also beware of problems due to noise, crosstalk from adjacent pins, and ground bounce. Also, the hottest spot on the chip may not necessarily be near the bond pad, and temperatures could be higher elsewhere. (DI #1555)